Law enforcement sources tell TMZ ... David Cocordan went to Judge Sautner's courtroom on Good Friday -- April 22 -- and informed the clerk he was Lindsay's fiance and needed to speak to the judge about her criminal theft case.

We're told Cocordan acted so bizarrely ... courthouse security contacted the police department in the city where the judge lives and asked them to do drive-bys at her home to make sure she was safe.

There's reason to fear Cocordan. Aside from the threatening and sexual messages he sent Lindsay and the order of protection she received, Cocordan was also arrested for robbery in 2004.